The present invention generally relates to a new and improved vibrating screen and method for dressing, sizing, desliming or dewatering feed streams of fine particulate material. More particularly, the present invention relates to a vibratory sieve bend screen and to a combination thereof with a planar screen deck to substantially improve the dressing desliming and dewatering of feed streams, such as solid/liquid slurry mixtures and to an improved method of performing such functions.
In the dewatering of solid/liquid slurry mixtures of fine grained particles, such as coal mineral ore, or the like, it is frequently the practice to use two separate pieces of apparatus. The first is a static sieve bend screen used for preliminary dewatering followed by a substantially horizontal vibrating screen deck which performs further dewatering of the feed stream. This technique is used extensively in the coal industry and a typical example of a coal preparation plant utilizing this system can be found in the text entitled "Coal Preparation" published by the American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., New York, (1979) at Chapter 8, pages 43-45 and Chapter 10, pages 7-11. Conventional sieve bends used in those techniques comprise a vertically disposed stationary concave screen made from a plurality of spaced bars arranged perpendicular to the direction of feed across the screen. The feed stream is applied to the top of the sieve bend and flows by gravity down the concave screen where thin laminae of the stream are removed as it progresses along the screen. The operation of these sieve bend screens is based on successive bars of the sieve bend engaging succeeding layers of the slurry mixture thereby removing water and particles from such layers to a depth of approximately one-quarter the aperture width. The particles passing through the apertures typically have a diameter of less than one-half the aperture width. The use of sieve bends is described in the publication "SME Mineral Processing Handbook" published in 1985 by the American Institute of Mining, Metallurgical and Petroleum Engineers. Reference may also be made to U.S. Pat. Nos. 3,353,674 and 4,512,880.
In some installations, it is known that the sieving action of the sieve bend screen is considerably enhanced by rapping or striking the underside of the screen to dislodge particles trapped in the apertures. In all known screens of this type, the principal motion imposed on the screen by the rapping or vibrating devices is normal or nearly normal to the direction of flow of the material across the screen. At the Coal Preparation Conference held as recently as May 1985, a paper was presented by David W. Brentz entitled "Mechanical Dewatering of 28.times.100 Mesh Fines On Vibrated Sieve Bends". As indicated in that paper, rapped sieve bends have been utilized to improve the removal of liquid and minus 100 mesh material. The rapped sieves unfortunately allow significantly high amount of oversized material in the effluent. Additionally, as stated in U.S. Pat. No. 4,519,902, the rappers on such sieve bends can cause destruction of the correlation between aperature spacing and particle size and the amplitude of vibration frequently is uneven over the entire length of the screen thereby reducing the efficacy of the vibration imposed on the screen.
Another disadvantage of the utilization of a static or rapped sieve bend in combination with a horizontal vibrating screen is that significant floor space and head room is required for such devices since the sieve bend is positioned well above the imput end of the horizontal vibrating screen thereby requiring head room for both pieces of equipment of about 15 to 20 feet. Additionally, the capital costs and maintenance costs associated with two separate pieces of equipment is significant. Further, although rapped sieve bends significantly overcome the clogging or blinding problem previously associated with the static devices, the mechanically rapped or vibrated sieves exhibit inefficiencies as dewatering devices. The rappers act on the sieve bend to provide a force component in a direction essentially normal or perpendicular to the screen surface thereby tending to force the entire slurry away from the separating action of the sieve bend deck. Additionally, such devices accelerate fatigue and wear of the screen.
To avoid these deficiencies, certain sieve structures such as that described in U.S. Pat. No. 4,519,902 impart a circular or elliptical vibratory motion to a floating sieve screen and frame so as to provide better distribution of the vibration over the entire screen and to circumvent the localized wear and fatigue associated with the rapping operation. Nevertheless, the vibratory motion employed is a non-linear elliptical motion with a throw that is essentially vertical.
Accordingly, it is an object of the present invention to provide a new and improved sizing and dewatering screening device and method having improved dewatering efficiency relative to the screens mentioned hereinbefore. Included in this object is the provision for a single vibratory screening device that utilizes a separate sieve screen and planar screening deck mounted in a common frame in a unique and efficient manner with the entire mechanism vibrated in unison and in a preselected linear straight line direction relative to the travel of the feed stream across the surface of the sieve screen. Advantageously, the device eliminates the need for separate frames and vibratory drives thereby lowering capital costs and maintenance while reducing the number of machines required to handle the same capacity.
Another object of the present invention is to provide a new and improved vibratory screening apparatus and method employing a concave or straight sieve screen wherein the vibratory motion imparted thereto is linear and is not in a direction normal to the screen surface. Included in this object is the provision for the application of vibratory motion that is effective to impede the flow along the sieve thereby increasing the retention time of particles on the sieve and substantially improving the dewatering performance of the device.
Still another object of the present invention is to provide a new and improved dewatering or classifying apparatus and method that imparts to the particles flowing therealong a significant reversal in the direction of flow and interruption of momentum from the sieve portion of the device to the planar deck portion thereof so as to momentarily impede and thereby further improve the retention time on the screening surface and enhance the efficiency of operation.
A further object of the present invention is to provide a new and improved screening apparatus of the type described that permits nesting of an arcuate or straight sieve within the principal vibratory screen frame so as to permit savings in floor space and head room thereby reducing plant costs and achieving more efficient and economical operation.
A still further object of the present invention is to provide a new and improved screening device of the type described wherein the sieve portion of the vibratory device may be adjustable so as to alter its angle of inclination within its supporting frame thereby permitting the operator to select an appropriate inclination resulting in optimum screening performance.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects and advantages are achieved in accordance with the present invention by providing a vibratory screening device for dressing, desliming and dewatering a stream of fine mineral particles flowing along the device. The device comprises a sieve screen having a midsectional zone for screening or dewatering the stream of particles and a discharge point at the downstream end of said midsectional zone. The device also includes a frame fixedly mounting the sieve screen for vibratory movement and linear vibrator drive members mounted on the frame for imparting straight line linear vibratory motion to the sieve screen, the linear motion being in a direction that imparts a counter current force to the particle flow at the midsectional zone of the sieve screen.
In a preferred embodiment, the device further includes a planar screen deck having a slightly upwardly inclined main deck portion extending from a feed end thereof and adapted for the vibratory conveyance of the mineral particles away from the feed end. The frame mounting the concave sieve screen also fixedly mounts the planar screen deck for common linear vibratory movement with the sieve screen in a direction tangential to the midsection of the sieve. The sieve screen and the screen deck are arranged so that the flow of particles along the sieve toward its discharge end is in one flow path direction while the particles deposited on the screen deck by the sieve flow in a direction generally opposite to that first flow path direction whereby the particles flowing along the sieve must undergo a reversal in direction of flow upon reaching the feed end of the deck.
The present invention further includes a method of vibratory screening that comprises the steps of feeding a particle stream to a vibratory sieve for gravity flow therealong and imparting straight line linear vibratory motion to the sieve in a direction substantially aligned with the flow across the midsectional zone of the sieve so as to impart to the particles thereon a desired principal force in a direction counter current to the flow of the particles and provide a first stage screening of the particle stream. Subsequently, the particles from the sieve are discharged therefrom and directed onto a planar vibratory screen deck for vibratory conveyance therealong in a direction generally opposite to the flow direction at the discharge end of the sieve screen so that a second stage screening of the particle stream takes place along the screen deck.
The sieve screen may be constructed so that the inclination thereof may be adjusted and the underflow from the sieve bend screen is separately collected for treatment apart from the underflow of the planar screen deck. The upward main deck portion of the planar screen deck is effective to partially resist the flow of the particles therealong and the common linear vibratory motion of the entire device is effective for advancing the particle stream towards the discharge end of the deck. In the preferred embodiment, the planar screen deck has an initial relatively short downwardly sloping portion and a relatively long upwardly inclined main deck portion arranged so that the linear vibratory motion of the device imparts an upward and forward movement to the particle stream flowing therealong. The combination is effective to provide both dewatering or screening of a particle slurry and its forward conveyance along the deck resulting in a particle cake at the discharge end of the deck that is essentially drip free.
A better understanding of this invention will be obtained from the following detailed description and the accompanying drawings which set forth illustrative embodiments indicative of the way in which the principles of the invention are employed including the several steps of the method and the relationship of one or more of such steps with respect to each of the others.